A cellular communication system projects a number of cells onto the earth. A frequency spectrum is allocated in frequency, in time, by coding, or a combination of these, to the cells so that communications taking place in nearby cells use different channels to minimize the chances of interference. On the other hand, communications taking place in cells located far apart may use the same channels, and the large distance between communications in common channels prevents interference. Over a large pattern of cells, a frequency spectrum is reused as much as possible by distributing common channels over the entire pattern so that only far apart cells reuse the same spectrum. An efficient use of spectrum results without interference.
One problem which cellular communications systems address is the handing-off of communications between cells, particularly between cells of different base stations or between different public land mobile networks (PLMNs) for terrestrial based cellular systems, and between cells of different satellites in satellite based cellular systems. Relative movements between end users and cells causes the end users and the communication links directed thereto to move between cells. In order to permit continuous communications in an ongoing call, the system must "handoff " the call when the end user (hereinafter "subscriber") crosses a cell boundary. If a call is not handed off to a new cell upon leaving an old cell, the call will eventually be lost because the strength of signals over which communications take place would diminish to a point where the subscriber's radio- telephone (hereinafter "subscriber unit") cannot receive the signal transmitted by the other terminating node, which may be another subscriber unit or a conventional telephone coupled to a public switched telephone network (PSTN), or vice versa.
Techniques exist for handing off calls between base stations in a terrestrial based cellular system, and between different terrestrial based cellular systems (i.e., between different PLMNs). Techniques exist for handing off calls between different satellites in a satellite based cellular system. Terrestrial based cellular systems do not provide coverage world wide. If a subscriber roams into uncovered territory during a call over a terrestrial based cellular system, the call eventually terminates due to loss in signal strength. Satellite based cellular systems provide much greater coverage, but are more costly. Accordingly, a need exists for providing interoperahility between terrestrial based cellular systems and satellite based cellular systems. In particular, a need exists for a method and system for allowing handoffs between terrestrial based cellular systems and satellite based cellular systems, including a dual-mode subscriber unit which permits a subscriber to communicate via either system.